Cure time, vulcanization

Table 6 Vulcanization Rate Constant (k) and Optimum Cure Time (T90)...

Vulcanization was carried out in one and two stages with and without carbon black. In one-stage vulcanization, all the ingredients were mixed and the compound was vulcanized in the mold for optimum cure time as shown by Monsanto rheometer curves. In the two-stage vulcanization process, all the ingredients except XNBR were mixed and heated for a short time, followed by mixing with required amount of XNBR and heating for rest of the time required for optimum vulcanization. 

The torque continues to increase, until there is no more signihcant rise. At this point the compound is vulcanized, and this maximum torque value is designated by the symbol Mh- The last major piece of data to be extracted is the time it takes to complete the cure, known as the cure time (t x). 

Influence of the ZnCFO contents (3,0 5,0 7,0 phr) on crosslink kinetics of the modelling unfilled rubber mixes from NBR-26 of sulfur, thiuram and peroxide vulcanization of recipe, phr NBR-26 - 100,0 sulfur - 1,5 2-mercaptobenzthiazole - 0,8 stearic acid - 1,5 tetramethylthiuramdisulfide - 3,0 peroximon F-40 - 3,0, is possible to estimate on the data of fig. 7. As it is shown, the increase of ZnCFO concentration results in increase of the maximum torque and, accordingly, crosslink degree of elastomeric compositions, decrease of optimum cure time, that, in turn, causes increase of cure rate, confirmed by counted constants of speed in the main period (k2). The analysis of vulcanizates physical-mechanical properties testifies, that with the increase of ZnCFO contents increase the tensile strength, hardness, resilience elongation at break and residual deformation at compression on 20 %. That is, ZnCFO is effective component of given vulcanization systems, as at equal-mass replacement of known zinc oxide (5,0 phr) the cure rate, the concentration of crosslink bonds are increased and general properties complex of rubber mixes and their vulcanizates is improved. 

The comparative estimation of efficiency of zinc oxide and ZnCFO similar concentrations (3,0 5,0 7,0 phr) as the agents of metaloxide vulcanization system was carried out on example of modelling unfilled elastomeric compositions from chloroprene rubber of recipe, phr chloroprene rubber - 100,0 magnesium oxide - 7,0. Kinetic curves of rubber mixes curing process at 155°C are shown on fig. 8. The analysis of the submitted data testifies, that at increase of zinc oxide contents vulcanization kinetics is changed as follows the scorch time and optimum cure time are decreased, the cure rate is increase. Vulcanization... 

Uses. Used in iron and steel production and in non-ferrous metals and alloys. It improves the creep strength of tin and the mechanical properties of lead. Used also in the vulcanization of rubber to reduce curing time and improve its characteristics. 

Measuring Vulcanization. The formation of a three-dimensional structure during vulcanization increases the stiffness (modulus) of the compound. Therefore, following the modulus increase versus cure time provides a continuous picture of the vulcanization process. Oscillating disk rheometers provide a useful method to do this (17). In this test, a preweighed sample of uncured mbber is placed into a preheated cavity containing a conical rotor. The cavity is closed and the rotor is set to oscillate within the mbber sample. As vulcanization proceeds, the compound s resistance to rotor movement increases and this resistance is followed as a function of time, thereby generating a continuous profile of cure behavior. These cure curves,... 

The rate of cure is the rate at which the cross linking and development of stiffness of the compound occurs after the scorch point. As the compound is heated past the scorch point, the properties of the stock change from a soft plastic to a tough elastic material. The rate of cure is an important parameter of vulcanization, since it, in part, determines the time the compound must be cured which is known as the cure time. 

Cure time is the time required for vulcanization of the rubber compound to reach the desired state of cure for the desired property. 

In all the above methods, it is necessary to cure specimens of test samples for each of a series of curing times and then perform the desired test on the vulcanizate. However, in the test for continuous measurement of vulcanization complete information could be obtained with saving in time. The mooney viscometer test approaches this objective. However a weakness of the mooney viscometer test is that the test is completed before a measurable modulus value after the scorch point has been obtained. This is because the test sample is destroyed after the induction period is passed due to tearing by continuous rotation of the rotor whether small or large. To overcome this deficiency and to provide a total cure curve for the entire vulcanization cycle, a series of instruments called cure meters was developed. In each of these instruments the stiffness or modulus of the compound was chosen as parameters for vulcanization continuously. The Vulkameter developed by Bayers, Germany was the first of the cure meters developed. 

The vulcanization temperature must be chosen in order to produce a well cured product having uniform and optimum physical properties in the shortest possible time. The term temperature coefficient of vulcanization can be used to identify the relationship between different cure times at different temperatures. With this information optimum cure times at higher or lower temperature can be estimated for many rubber compounds with known coefficient of vulcanization. For approximately most rubber compounds the coefficient of vulcanization is 2. This indicates that the cure time must be reduced by a factor of 2 for each 10°C increase in cure temperature or if the temperature is reduced to 10°C, the cure time must be doubled. 

This is due to over-vulcanization. The remedy is to reduce cure times and temperatures or modify the compound formulation to reduce the speed of cure. Tearing can also be due to method of removal of the product from the mould while it is in hot condition. Tearing can be eliminated by removing the products from the mould after sufficient cooling or by careful and slow removing. 

Cure time - The time required to produce vulcanization at a given temperature. (Cure time varies widely, being dependent on the type of compounding used and the thickness of the product.)... 

Some polysiloxanes are curable with lead monoxide, with a consequent reduction in both curing time and temperature. High-frequency electrical energy vulcanizes in one case at least. Zirconium naphthenate imparts improved resistance to high temperatures. Barium salts are said to prevent blooming. Sulfur dichloride is also used. Some resins are solidified by pressure vulcanization, using di-f-butyl peroxide. Improvements are to be found in lower condensation temperatures and shorter times of treatment... 

Mooney viscosity—measure of the resistance of raw or unvulcanized rubber to deformation, as measured in a Mooney viscometer. A steel disc is embedded in a heated rubber specimen and slowly rotated. The resistance to the shearing action of the disc is measured and expressed as a Mooney viscosity value. Viscosity increases with continued rotation, and the time required to produce a specified rise in Mooney viscosity is known as the Mooney scorch value, which is an indication of the tendency of a rubber mixture to cure, or vulcanize, prematurely during processing. 

Several parameters of interest were considered in this work the one concerned with the operational conditions of the rubber in the storage bulb as time and temperature, the other with the working conditions in the mold as mold temperature, cure time, thickness of rubber sheets, cure enthalpy (or % vulcanizing agent). 

In the one-stage vulcanization process, NR and EPDM are first masticated separately and then mixed with each other. Additives such as ZnO, stearic acid, carbon black (or silica), and process oil are added. The mix thus obtained is allowed to cool to room temperature. Finally, coupling agent known as DIPDIS and sulfur are added to the mix on the cooled mill. The stocks are finally cured under pressure at 160°C (32-33). In the two-stage process, NR and EPDM are first masticated separately. Then, additives such as ZnO, stearic acid, DIPDIS, and sulfur are incorporated in the EPDM. The compounded EPDM mix is then heated at 160°C in the hydraulic press for the predetermined time to yield the grossly undercured mix. The undercured mix is then blended with NR to the required blend ratio. The blend compound is finally vulcanized to the optimum cure time values (32-33). 

Rubber compounders also learned to live with a certain number of failures. Accelerators and other additives which reduce curing time and prevent sulfur blooming were developed. The possible effects of the various allotropic forms of sulfur in rubber have been reported (18, 19, 20). Although too costly to have gained wide commercial acceptance, organic sulfur compounds make excellent vulcanizing agents for rubber (21). Related compounds cure bituminous mixtures (22). 

Figure 3 shows typical rheometer cure time curves of three Blon elastomers with different crosslinker levels at 50 phr carbon black loading. With higher crosslinker content In the polymer, the torque required to shear the rubber during vulcanization Increased while cure time decreased. 

The cure of vulcanization of elastomers is a process whereby chemical crosslinks are introduced between the elastomer chains, resulting in the formation of a three-dimensional network. Overall, the chemistry accompanying snlfnr vulcanization is extremely complex and althongh several techniqnes have been developed to monitor the process and to analyze the prodncts formed, most are time-consnming and tedious. 

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